Apply power to these dampers and the Normally Open Dampers will close shutting off all airflow. Apply power to Normally Closed Dampers and they will open allowing air to pass. Remove power and a spring returns the dampers to their original position. These new dampers have a specially designed seal that ensures no air leakage when closed to prevent energy loss and wind noise. They are great for preventing CO2 and odors from escaping your grow area. Available in 4 different duct sizes. Power supply included.